CS1315: Introduction to Media Computation “What do other languages look like? How hard is it to do Web programming?”: JavaScript

What do other languages look like?  

We call the language “look” its syntax Python is a fairly traditional language in terms of syntax. 



Languages like Scheme and Squeak are significantly different.

Major points of difference:   

Whether or not variables have to be declared before first use. Details of how individual lines are written. Details of how blocks are defined.

JavaScript 

JavaScript is meant to be a scripting language, like Python.

Scripting languages are meant for non-professional programmers to solve simple tasks.  It’s designed to look like Java to ease the transition in either way. 



JavaScript can be used by the web server (used on the computer accessed via the Internet), or it can be used within an HTML page.

If it’s within the HTML page, it’s actually executed by the user’s browser.  We call that client side JavaScript. 

JavaScript syntax: Variables 

Variables must be declared before use.   



You can’t just say: a = 12 You can either say: var a = 12; Or: var a; a = 12

In other languages, you might also declare the variable’s type int a=12;

JavaScript syntax: Blocks 

Blocks are delimited with curly braces.

function test() { document.writeln("This is a test"); }

JavaScript syntax: Individual statements 

Lots of differences:  

func tion instead of def End lines with semicolons “;” 

 



(But lines can have returns in the middle of them.)

The for statement is numeric (mostly) and has different parts to it. You use write or writeln instead of pri nt

But they’re mostly detail changes. 

The basic operation of JavaScript is not unlike Python.

JavaScript is all about objects   

Just about every function is actually a method. For example, there is no global print. There is a function write or writeln 



Writeln adds a new line (‘\n’) at the end.

But these aren’t global functions.  

To write into the document, you use document. wri te() document. wri te() is a method on the HTML document itself.

Embedding JavaScript inside HTML 

JavaScript sits inside of HTML pages. 



You wrap <script> tags around the JavaScript.

You can have <script> tags in two kinds of places.  

Inside the tags to define functions used elsewhere. Inside the body, where the scripts are actually executed.

Our Simple Web Page

A Simple Heading

This is a very simple web page.

Adding some simple JavaScript <script> function test() { document.writeln("This is a test"); }

A Simple Heading

This is a very simple web page.

<script> test()

Going into detail on the function <script> function test() { document.writeln("This is a test"); }

A Simple Heading

This is a very simple web page.

<script> test()

Here’s a function  named “test” with  no inputs, that only  writes out a string.

Here we  execute the  function.

Can also insert HTML <script> function insertHead() { document.writeln("

This is a test

"); }

A Simple Heading

This is a very simple web page.

<script> insertHead()

Using loops <script> function countToTen() { document.write("

"); for (i=1; i<= 10; i++) { document.write("
• Item number: ",i); } document.write("

"); }

A Simple Heading

This is a very simple web page.

<script> countToTen()

Explaining that function We can write  function countToTen() out 

 and  {

document.write("

"); for (i=1; i<= 10; i++) { document.write("
• Item number: ",i); } Creating an item for  document.write("

"); each value of i }

Explaining that Loop for (i=1; i<= 10; i++) { document.write("

Item number: ",i); }



A for loop has three parts, separated by semi-colons. 

What to do first. 



When to stop 



For us, i<=10

What to do each time through the loop 



For us, set i to 1

i++ means to increment the value of i by 1.

It’s a notation that was invented by the programming language C and has been adopted by many languages

Operators in JavaScript 

The same kind of operators you know in Python 

+-*/ 

  



+ even works for strings, as well as numbers

< <= > >= == and != ! for not

Logical operators are a little weird  

&& is AND || is OR

Can we display anything useful? 

Sure!  

Anything you can compute. Anything that you can get from builtin functions (mostly methods). 



There are lots of them.

You don’t have to have a function either. 

You can just put the script in-line

Displaying the date and time

This is a very simple web page.

This is being served to you on <script>document.write(Date() );

Using dialogs in JavaScript

function check() { var agree = false; agree = confirm('Do you enjoy CS?'); if (agree) agree will be true or  notes=prompt("Give me one good thing about CS:"); if (! agree) false.   notes=prompt("Why don't you like CS?"); alert("You said:"+notes); ! agree is not agree. } … <script> check() Notice: We can indent or not  indent as we want here.  

Indentation is not important  in JavaScript (or most other  languages.)

What happens when this runs

Different kinds of dialogs  



Confirm: Puts up a prompt, returns true or false. Alert: Beeps and displays one thing with an OK button. No return. Prompt: Asks the user for a line of text. Returns that text.

Running on Loading the Page  

This program runs when the page loads. Is that what you really want to happen? 



The user sees nothing at all until they go to your page and then these dialog boxes happen.

Isn’t it more natural for dialog boxes to pop up when you click on something?

Events: Key to responding to users  



The key to responding to users are events. Events are actions taken by the user that can be caught by your program in JavaScript. Events are caused by the user typing a key, moving the mouse, clicking the mouse.

Events in JavaScript        

onKeyPress onKeyUp onKeyDown onClick onDblClick onMouseOver onMouseOut onMouseMove

 

onMouseDown onMouseUp

Catching an event 

In the anchor or image tag, assign the event to some JavaScript code in a string.

Example Event Code

A Simple Heading

This is a very simple web page.